Field of the Invention (Technical Field)
The claimed invention relates to centrifugal fan design and control, and more particularly to a system, method, and computer program to a control system for a backward inclined centrifugal fan assembly to allow for a wider range of operation of the fan system by controlling the effective width of the fan wheel.
Background Art
Backward inclined centrifugal fans are typically selected for maximum static efficiency when operating at design conditions. This results in the lowest power requirement, as the fan is operating near the stall region. However, if the fan system needs to operate a reduced flow rate, like those found in variable air volume distribution systems in office buildings, university buildings, etc., then, the fan is prone to stalling at a relatively high percentage of the design volume (cubic feet per minute (CFM)).
There are adverse effects to operating a backward inclined centrifugal fan system in the stall region such as excessive vibration in the fan and attached systems, and in extreme cases, mechanical failure of the fan assembly.
To overcome this limitation when selecting a fan system, it may be necessary to select a fan with a lower than desired peak static efficiency to allow for the required reduction in flow rate. This results in a higher peak power requirement for the fan system than would otherwise be possible. To select a fan with a lower peak static pressure, some possible methods are to select larger diameter fans at a slower speed, or to reduce the width of the fan wheel. Some issues with these approaches are that larger fans may not fit in the required space and are more expensive, and reducing the width of the fan wheel causes reduction in the peak efficiency of the fan.
An alternate method for reducing the air flow is to design a fan system consisting of multiple individual fan assemblies in parallel to allow for shutting down individual fans when low flow rates are required. This means using many smaller fans to allow for the large volume turndowns required of large fan systems. Smaller fans are generally less efficient than larger fans of the same design, so this again results in increased power consumption when running at design conditions. This method also limits the ability to provide a smooth delivery of air when the fans are cycled on and off.
An ideal solution to this problem is prevention of fan stall when operating at low volume deliveries and elevated static pressures, while maintaining a high efficiency system. The presently claimed invention allows for easy field tuning, positive shutoff, and monitoring of valve position during operation.